1. Field of the Invention
The present invention relates to an endoscope and an endoscope apparatus having an image pickup apparatus including two image pickup devices.
2. Description of the Related Art
Image pickup apparatuses having an objective optical system and a solid-state image pickup device (hereinafter referred to as a CCD) have been widely used. The image pickup apparatus which can obtain high quality images from picked-up observed image is naturally desired and for some apparatuses in which the image pickup apparatus is installed, the image pickup apparatus is desired to have a reduced size.
In particular, in an endoscope having the image pickup apparatus in a distal end portion of an insertion portion, although the structure of the distal end portion varies among different types of endoscopes, mostly the image pickup apparatus and a light guide occupies a large part of the volume of the space in the distal end portion. Consequently, the sizes of these two members substantially determine the diameter of the distal end portion of the endoscope. Thus, a reduction in the size of the image pickup apparatus enables a reduction in the diameter of the distal end portion of the insertion portion and in the diameter of the insertion portion itself.
A part of the image pickup apparatus in which the CCD is mounted has the largest size in the whole image pickup apparatus. Consequently, determining the size of the CCD substantially determines the diameter of the distal end potion of the insertion portion. Therefore, for a reduction in the size of the image pickup apparatus, the configuration of the part with the CCD mounted therein must be taken into account.
Further, as described above, the image pickup apparatus needs not only to have a reduced size but also to be able to pick up high-quality images. A common method for improving the quality of images picked up by the image pickup apparatus is to increase the number of pixels in the CCD.
However, an increased number of pixels necessarily increase the size of the CCD, making it difficult to reduce the diameter of the distal end portion of the insertion portion. That is, there is a tradeoff relationship between the reduced diameter of the distal end portion and the improved image quality.
A well-known method for achieving both the reduced diameter of the distal end portion and the improved image quality is a multiple CCD structure in which a plurality of CCDs are used to improve the image quality. With the diameter of the distal end portion adjusted to be equivalent to that in a single CCD structure, an image pickup apparatus with a double CCD structure using two CCDs (hereinafter referred to as a double image pickup apparatus) is suitable for the endoscope.
For example, a single-plate image pickup apparatus with a single CCD structure using a single CCD has color filters for red, green, and blue or cyan, magenta, and yellow in the CCD so that colors can be formed using four pixels.
On the other hand, in a double image pickup apparatus using two CCDs, one of the CCDs is equipped with a prism coated so as to reflect green, while allowing red and blue to pass through and red color filters and blue color filters provided in stripes in a direction in which red and blue pass through the filters. The other CCD has black and white color filters or green color filters in a direction in which green is reflected. Thus, colors are formed using two pixels in the CCDs. That is, the image quality can be improved without the need to increase the number of pixels.
A conventional technique for such a double image pickup apparatus is disclosed in, for example, Japanese Patent Laid-Open No. 2000-221411 or No. 61-129961. Japanese Patent Laid-Open No. 2000-221411 discloses a technique for an electronic endoscope apparatus having a double image pickup apparatus with two CCDs; Japanese Patent Laid-Open No. 2000-221411 intends to provide an image pickup apparatus which provides a higher quality than a single-plate image pickup apparatus with one CCD and which is more inexpensive than a triple image pickup apparatus with three CCDs.
Japanese Patent Laid-Open No. 61-129961 discloses a technique for a double solid-state image pickup apparatus for an electronic still camera which is configured as a double image pickup apparatus described above and specifically in which one of the CCDs is located parallel to an incident optical path, whereas the other CCD is located perpendicular to the incident optical path.